Is it ice cream, ice-cream, or icecream? What about mother in law or mother-in-law? Compound nouns combine two or more words to create a new meaning, but their spelling can be confusing. This guide covers all the rules you need to write compound nouns correctly.
What is a Compound Noun? A compound noun is formed when two or more words join together to create a single noun with a new, specific meaning. The meaning is often different from the individual words: black + bird = blackbird (a specific bird species, not just any black bird).
The Three Types of Compound Nouns
Compound nouns can be written in three different ways:
| Type | Format | Examples |
|---|---|---|
| Closed (Solid) |
One word no spaces |
toothpaste, bedroom, football, sunflower, keyboard, moonlight, earthquake, notebook |
| Hyphenated | Words joined with hyphen(s) |
mother-in-law, well-being, six-pack, merry-go-round, check-in, self-esteem |
| Open (Spaced) |
Separate words with space |
ice cream, post office, real estate, high school, living room, bus stop |
💡 The Challenge: There's no single rule that determines which type to use. The spelling often depends on how long the compound has existed (older compounds tend to become one word) and common usage. When in doubt, check a dictionary!
Word Combination Patterns
Compound nouns can be formed from different combinations of word types:
| Pattern | Examples | More Examples |
|---|---|---|
| Noun + Noun | tooth + paste = toothpaste bus + stop = bus stop |
bedroom, football, sunrise, haircut, bookshelf, website |
| Adjective + Noun | black + board = blackboard soft + ware = software |
greenhouse, highway, full moon, hot dog, small talk |
| Verb + Noun | swim + suit = swimsuit wash + room = washroom |
cookbook, driving license, washing machine, rainfall |
| Noun + Verb | sun + rise = sunrise hair + cut = haircut |
rainfall, snowfall, footprint, heartbeat |
| Verb + Preposition | break + down = breakdown check + out = checkout |
takeoff, makeup, workout, input, output |
| Preposition + Noun | under + ground = underground over + time = overtime |
onlooker, bystander, afterthought, upstairs |
| Noun + Prepositional Phrase | mother + in + law = mother-in-law | father-in-law, editor-in-chief, commander-in-chief |
Spelling Guidelines by Category
Usually Written as ONE WORD (Closed)
| Category | Examples |
|---|---|
| Body parts + noun | toothbrush, toothpaste, haircut, eyebrow, eyelid, fingertip, footprint, heartbeat |
| Time + noun | daytime, nighttime, weekend, weekday, lifetime, birthday |
| Nature words | sunlight, moonlight, starfish, earthquake, waterfall, rainbow, snowflake |
| Room/place + noun | bedroom, bathroom, classroom, airport, seaport, sidewalk |
| Common everyday items | newspaper, keyboard, laptop, smartphone, backpack, textbook, notebook |
| -thing, -one, -body, -where | something, someone, somebody, somewhere, anything, anyone, everybody, nowhere |
| Sports | football, basketball, baseball, volleyball, skateboard |
Usually Written with HYPHENS
| Category | Examples |
|---|---|
| Family relationships with "in-law" | mother-in-law, father-in-law, sister-in-law, brother-in-law, son-in-law, daughter-in-law |
| Compounds with "self" | self-esteem, self-control, self-confidence, self-awareness, self-service |
| Numbers + nouns | two-year-old, six-pack, ten-speed, five-star |
| Titles with "in-chief", "elect" | editor-in-chief, commander-in-chief, president-elect |
| Well- / ill- / half- compounds | well-being, ill-treatment, half-brother, half-sister, half-price |
| Verb-preposition as nouns | check-in, check-out, break-in, run-up, warm-up, mix-up, grown-up |
| Direction compounds | north-east, south-west (British) or northeast, southwest (American) |
Note: Many hyphenated compounds are evolving toward closed forms. E-mail became email, on-line became online. Check a current dictionary for the latest preferred spelling.
Usually Written as TWO WORDS (Open)
| Category | Examples |
|---|---|
| Food and drink | ice cream, hot dog, peanut butter, orange juice, French fries, potato chips |
| Places and buildings | post office, police station, fire station, bus stop, train station, shopping center |
| Academic terms | high school, middle school, graduate school, law school, medical school |
| Rooms in a house | living room, dining room, laundry room, waiting room |
| Professional terms | real estate, human resources, social media, public relations, customer service |
| Swimming + noun | swimming pool, swimming lesson |
Common Compound Nouns: Quick Reference
| One Word ✓ | Hyphenated ✓ | Two Words ✓ |
|---|---|---|
| aircraft | check-in | air conditioning |
| airport | cross-reference | alarm clock |
| backstage | dry-cleaning | blood pressure |
| basketball | editor-in-chief | bus stop |
| bathroom | father-in-law | credit card |
| bedroom | half-brother | dining room |
| bookstore | life-size | fire engine |
| breakfast | merry-go-round | first aid |
| classroom | mother-in-law | French fries |
| cupboard | runner-up | high school |
| downtown | self-control | hot dog |
| earring | self-esteem | ice cream |
| earthquake | six-pack | living room |
| everybody | T-shirt | middle class |
| firefighter | warm-up | parking lot |
| football | well-being | peanut butter |
| grandmother | x-ray | police officer |
| haircut | post office | |
| homework | real estate | |
| keyboard | swimming pool | |
| laptop | text message | |
| moonlight | traffic light | |
| newspaper | washing machine | |
| sunglasses | water bottle | |
| toothpaste | web page |
Plural Forms of Compound Nouns
Forming plurals of compound nouns follows specific patterns:
| Rule | Singular → Plural | More Examples |
|---|---|---|
| One-word compounds: Add -s/-es at the end |
bedroom → bedrooms toothbrush → toothbrushes |
keyboards, earthquakes, cupboards, girlfriends |
| Open compounds: Pluralize the main noun |
bus stop → bus stops ice cream → ice creams |
swimming pools, police officers, post offices |
| Hyphenated with main noun first: Pluralize the main noun |
mother-in-law → mothers-in-law passer-by → passers-by |
sisters-in-law, editors-in-chief, runners-up |
| Hyphenated with no clear noun: Add -s at the end |
grown-up → grown-ups check-in → check-ins |
warm-ups, mix-ups, break-ins |
Correct Plurals:
One mother-in-law → Two mothers-in-law ✓
One attorney general → Two attorneys general ✓
One court martial → Two courts martial ✓
I have two mother-in-laws.
I have two mothers-in-law.
Possessive Forms of Compound Nouns
Add 's to the end of the compound, regardless of type:
| Compound Type | Possessive Form | Example Sentence |
|---|---|---|
| One word | girlfriend's | My girlfriend's car is red. |
| Hyphenated | mother-in-law's | My mother-in-law's house is nearby. |
| Open | post office's | The post office's hours changed. |
| Plural compound | mothers-in-law's | Both mothers-in-law's opinions matter. |
British vs American Spelling
Some compound nouns are spelled differently in British and American English:
| American English | British English | Notes |
|---|---|---|
| airplane | aeroplane | Spelling |
| cellphone | mobile phone | Different word |
| checkbook | cheque book | Spelling + spacing |
| northeast | north-east | Hyphen use |
| percent | per cent | Spacing |
| lineup | line-up | Hyphen use |
| healthcare | health care | Spacing |
Evolution of Compound Spelling
Compound nouns often evolve through three stages over time:
Stage 1: Open → Stage 2: Hyphenated → Stage 3: Closed
electronic mail → e-mail → email
web site → web-site → website
on line → on-line → online
data base → data-base → database
lap top → lap-top → laptop
Why does this happen? As compound words become more common and familiar, writers naturally want to join them more closely. What starts as two separate concepts becomes one unified idea—and the spelling reflects that.
Words That Look Similar But Differ
Be careful with these pairs—spacing changes the meaning:
| One Word | Two Words | Difference |
|---|---|---|
| everyday | every day | everyday (adj) = ordinary, common every day = each day |
| anyone | any one | anyone = any person any one = any single item |
| everyone | every one | everyone = all people every one = each individual thing |
| sometime | some time | sometime = at an unspecified time some time = a period of time |
| maybe | may be | maybe = perhaps may be = might be |
| into | in to | into = entering in to = in + infinitive |
I wear my everyday clothes. (ordinary clothes)
I exercise every day. (each day)
Maybe she'll come. (perhaps)
It may be too late. (might be)
Common Mistakes to Avoid
Mistake 1: Wrong spacing/hyphenation
I need to buy tooth paste.
I need to buy toothpaste.
My motherinlaw is visiting.
My mother-in-law is visiting.
Mistake 2: Wrong plural form
Both of my sister-in-laws came.
Both of my sisters-in-law came.
The passer-bys didn't help.
The passers-by didn't help.
Mistake 3: Confusing similar compounds
I do this everyday.
I do this every day.
This is my every day routine.
This is my everyday routine.
Summary
Compound noun spelling follows these key principles:
- Closed (one word): Common, established compounds → bedroom, football, toothpaste
- Hyphenated: Family-in-law, self- compounds, numbers → mother-in-law, self-esteem, six-pack
- Open (two words): Food, places, newer compounds → ice cream, post office, high school
- Plurals: Add -s to the main noun → mothers-in-law, passers-by
- Possessives: Add 's to the end → mother-in-law's
- Evolution: Compounds tend to close up over time → e-mail → email
💡 Pro Tip: When you're unsure about compound spelling, consult a current dictionary. Language evolves, and what was hyphenated yesterday might be one word today. Merriam-Webster (American) and Oxford (British) are reliable resources. Also, be consistent—pick one style and stick with it throughout your writing!
Tags
compound nounsnounsspellinghyphensgrammarintermediate
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